Shin-Etsu Chemical Co., Ltd.

Yoshihrio Shirota

Tadashi Amano

Yoshitaka Okuno

Fumiaki Adachi

This process uses such a polymerization apparatus that the inner volume of the polymerization vessel is 100 m.sup.3 or more, the ratio L/D of the length L of the straight barrel to the inner diameter D is 1.5 or more, and the uppermost stirring impeller is arranged in the range of 150 to 300 cm downwardly from the upper end position of the straight barrel of the polymerization vessel in terms of the height of the center line of the vertical width of the blade, and is characterized in that after the liquid contents of the polymerization vessel are heated to a temperature near the polymerization temperature, noncondensable gases coming into the reflux condenser are evacuated under such conditions that the flow rate of the evacuation is 10 to 100 Nm.sup.3 /hour and the evacuation time is a period equal to 20% or more of the total polymerization time, and simultaneously with or after the start of the evacuation of the noncondensable gases, the cooling by the reflux condenser is started. According to the present invention, even if a long-term operation is carried out by using a large-sized polymerization vessel, since the quantity of heat dissipation by the reflux condenser can be kept stable, and scale can be prevented from being deposited on the polymerization vessel inner wall and the like.

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Kamisu-machi, Japan

Yoshihrio Shirota